The present invention relates to an exhaust gas cleaner for effectively removing nitrogen oxides from an exhaust gas containing nitrogen oxides and an excess proportion of oxygen, and a method for removing nitrogen oxides with such an exhaust gas cleaner.
Various exhaust gases discharged from internal combustion engines such as automobile engines, etc., combustion apparatuses installed in factories, home fun heaters, etc. contain nitrogen oxides such as nitrogen monoxide and nitrogen dioxide together with an excess proportion of oxygen. The term "containing an excess proportion of oxygen" means that the oxygen content is larger than its stoichiometric proportion relative to unburned components such as carbon monoxide, hydrogen, hydrocarbons in the exhaust gas. The term "nitrogen oxides" means nitrogen monoxide and/or nitrogen dioxide.
The nitrogen oxides are one of causes of acid rain, posing a serious problem of environmental pollution. For these reasons, various methods have been proposed to remove nitrogen oxides from exhaust gases emitted from various combustion equipment.
In the case of large, stationary combustion apparatuses such as large combustion apparatuses of factories, ammonia is employed to catalytically and selectively reduce nitrogen oxides in an exhaust gas containing an excess proportion of oxygen, thereby removing nitrogen oxides from the exhaust gas.
However, such a method is disadvantageous, because ammonia is expensive, because ammonia is so toxic that the proportion of ammonia should be controlled by measuring the concentration of nitrogen oxides in the exhaust gas, and because this reduction system generally needs large apparatuses.
There has been proposed a method in which gases serving as a reducing agent, such as hydrogen, carbon monoxide, hydrocarbons etc. are brought into contact with nitrogen oxides, whereby the nitrogen oxides are non-selectively and catalytically reduced. In this method, the reducing agent must be added to the exhaust gas in a greater proportion than a stoichiometric proportion relative to oxygen in the exhaust gas to effectively remove nitrogen oxides from the exhaust gas, leading to an increased consumption of the reducing agent. For this reason, the method is applicable merely to the exhaust gas containing a small proportion of residual oxygen such as those generated almost under a theoretical air fuel ratio, resulting in limited application of the method to a narrow range of exhaust gases.
There have also been proposed methods of reducing nitrogen oxides by adding to an exhaust gas hydrocarbons in a smaller proportion than a stoichiometric proportion relative to oxygen in the exhaust gas, in the presence of a catalyst such as zeolite with or without carrying a transition metal (Japanese Patent Laid-Open Nos. 63-100919, 63-283727 and 1-130735; Thesis 2A526, 1990, the 59th Spring Conference of the Japan Chemical Society; Theses 3L420, 3L422 and 3L423, 1990, the 66th Catalysis Symposium, the Catalysis Society of Japan, 1990; and "Catalyst", Vol. 33, No. 2, p.59 (1991)).
However, these methods are effective only for removal of NOx having a narrow temperature range. Also, their efficiency of removing nitrogen oxides is extremely low in the case of an actual exhaust gas because it contains a moisture.